The present invention relates to a multiple wire printed circuit board using wires having insulating coating as circuit conductors, and a process for making such a circuit board.
Multiple wire circuit boards having an adhesive layer provided on a substrate, with the insulating layer coated wires for forming conductor circuits being bonded to said adhesive layer and with the respective layers interconnected by through-holes, are disclosed in U.S. Pat. Nos. 4,097,684, 3,646,572, 3,674,914 and 3,674,602, and known as printed wiring boards which are capable of high-density wiring and advantageous for matching of characteristic impedance and reduction of crosstalks.
The processes for making multiple wire circuit boards having an adhesive layer composed of a thermosetting resin, a curing agent and a rubber, disclosed in the above-mentioned U.S. patents, comprise essentially the steps of 1 forming inner layer circuits, 2 laminating an adhesive, 3 wiring, 4 laminating a prepreg, 5 drilling and 6 copper plating. Lamination of a layer of prepreg in step 4 is necessary for fixing the insulating layer coated wires in the substrate for preventing them from coming off during drilling or for preventing the insulating coating layers of wires from suffering damage to reduce reliability in the plating step for forming a metal layer in the holes.
Use of a rubber material for the adhesive layer is necessary for the following reason. Since an adhesive agent is applied on a support film and dried to form an adhesive sheet and it is laminated and bonded on an insulated substrate or inner layer circuit board having a layer of prepreg laminated thereon, it is required that the adhesive layer can be formed as a film, and that it has flexibility and can remain non-adhesive except during wiring for facilitating handling of the adhesive layer in the circuit board making process.
Further, in view of the fact that when fixing the insulating layer coated wires to the adhesive layer, said wires are brought into contact with the adhesive by the end of a stylus which is vibrated by ultrasonic waves, and the adhesive is activated and fused by the heat energy generated by the ultrasonic vibrations, the adhesive is required to be made of a fusible composition.
In the prior art, insulation resistance can be confined within the permissible tolerance if the wiring density is in the conventional range. Also, with reference to positional accuracy of wires, although there has been observed a positional deviation of wires (hereinafter referred to as "wire swimming") of up to about 0.2 mm from the design value after wiring and laminating of the prepreg layer, the conventional wire setting could withstand practical use because of low wiring density and large hole size.
However, if wiring density is increased as mentioned above, insulation resistance lowers sharply where an adhesive comprising a rubber material is used. Also, as the wiring density increased with consequent diminution of through-hole size and enlargement of wire swimming, there arose the problem of improper connection due to shift of the insulating layer coated wires at the locations where through-holes are to be formed.
The cause of this reduction of insulating resistance and enlargement of wire swimming was attributable to use of a rubber material as an adhesive component. That is, low insulation resistance of the rubber material itself and the fact that a layer of prepreg has been laminated and bonded with fluidity of the rubber material in the adhesive being retained after wiring, were to blame for the occurrence of wire swimming.
For overcoming this problem, an adhesive comprising a phenoxy resin, an epoxy resin, a curing agent, a reactive diluent and an electroless plating catalyst has been developed as an adhesive sheet, as disclosed in JP-B 2-12995. According to this development, reduction of insulation resistance is prevented by use of a polymer with high insulation resistance in place of the rubber material in the conventional adhesive.
In the prior art, prepreg pressing has been carried out after fixing of insulating layer coated wires. However, when this prepreg pressing is conducted using said adhesive after wiring, there takes place wire swimming to an excess extent, so that a heating step is incorporated between the wiring step and the pressing step to induce a slight degree of curing of the adhesive layer and to thereby inhibit wire swimming. Further, this adhesive had the problem of low solder heat resistance because of large amount of residual solvent.
Recently, increase of structural layers and miniaturization of printed wiring boards including multiple wire circuit boards are impelled to adapt them to high-density packaging. For implementing such increase of structural layers and miniaturization of multiple wire circuit boards, efforts have been made for increasing wiring density and for increasing and reducing size of the wiring layers. However, the multiple wire circuit board using the adhesive disclosed in JP-B 2-12995 involved the problem that solder heat resistance tends to lower as the wiring density increases. It was found that the breakage of conductors and wires was associated with the insulating layer coated wires and overlying layer of prepreg, or with the adhesive, and later, by providing an adhesive layer (covering adhesive layer) on the encapsulated wires for improving solder heat resistance, the wiring boards have become capable of practical use. However, provision of this covering adhesive layer has added to the number of steps and necessitated a longer time for completion of the process.
It was further found that multiplication of the wiring layers leads to a greater reduction of solder heat resistance than when only two wiring layers are provided. For instance, in a multiple wire circuit board having four wiring layers, there tends to take place peel or formation of voids in the adhesive layers on the inside two of the four wiring layers due to heat history. This phenomenon tends to occur in proportion as the through-holes piercing the multiple wire circuit board or the non-piercing via-holes increase, and it occurs typically in the adhesive layer.
A similar phenomenon is also observed in the multi-layer circuit boards, and it is known that such a phenomenon occurs in the layers containing no reinforcement such as glass fabric or nonwoven fabric in the multi-layer circuit boards.